In typical commercial electrographic reproduction apparatus (copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly, utilized in typical reproduction apparatus, includes at least one heated roller and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
Further, with fuser assemblies of the above described type, it has been found that there is a tendency of a portion of the marking particles in an image to adhere to the pressure roller rather than remaining with the receiver member during the fusing operation. This is referred to as image offset. Thereafter the offset marking particles can transfer back to subsequent receiver members being fused to form undesirable image artifacts such as ghost images for example. Also, the offset marking particles may transfer to the fusing roller when no receiver member is present therebetween and then to the back-side of subsequent receiver members to form undesirable marks thereon. In order to minimize this image offset effect, an offset preventing oil is applied to the rollers of the fuser assembly. The offset preventing oil is preferably of a high viscosity which has been found to lower the surface energy of the rollers and makes it less likely that marking particles will adhere to the rollers.
The oiling rate for fusing roller offset preventing oil, during the fusing process, is very important in order to prevent image quality problems by ensuring proper release of receiver members and the presence of a proper amount of offset preventing oil on the roller for maintaining optimum roller material life. It has long been desired to provide a system whereby it is ensured that the fusing roller oiling system is working and delivering the proper amount of oil to the fusing roller. However, typical flow sensors and the like have not been satisfactorily usable due to the high viscosity, and very slow movement, of the offset preventing oil.